November 20.doc

Phys 101 Alanna Houston
November 20, 2007
- Primarily see virtual image
- In the example of a real image, you would only see the image
clearly if you were further from the mirror than the image (so your
eye received diverging rays)
- Concave shapes can also be used for other waves (sound mirror)
- Convex mirrors can also use the mirror equation. Focal length is: f
= r/2
- Careful with the sings of distances: f is negative in this case (but
the rest of our sign convention is unchanged)
- A convex right-hand rear view mirror on a car. Radius of curvature
is 16.0 m. What is the image location and magnification for an
object 10.0 m away? Most mirrors like this carry a warning:
“objects in this mirror may be closer than they appear!” Why?
o r = 16.0 m
o f = -8.0 m
o 1/do + 1/di = 1/f
o 1/di = -1/8 – 1/10
o -4.44 m = di
o m = -4.4 / 10 = +0.44
- You look into a shiny Christmas tree ball (diameter 9.0 cm) from
30.0 cm away. Where is your image? Is it real or virtual? Upright of
inverted?
o 1/do + 1/di = 1/f
o r = 4.5 cm
o f = -2.25 cm
o –1/2.25 = 1/30 + 1/di
o di = -2.09 cm = 2.1 cm
o m = -di / do = - -2.1/3.0 = 1/15 = upright
o therefore, virtual and upright
THIN LENSES
- made up of spherical surfaces where the radius of curvature is
large compared to the lens size so we ignore spherical aberration:
that is, we assume that parallel rays are focussed at the focal
point at the focal length f from the lens: look at picture
- look at pictures of converging and diverging lenses.
- With ray tracing, we can draw the position of the image